Block of squid axon K channels by internally and externally applied barium ions

We have studied the interactions of Ba ion with K channels. Ba2+ blocks these channels when applied either internally or externally in millimolar concentrations. Periodic depolarizations enhance block with internal Ba2+, but diminish the block caused by external Ba2+. At rest, dissociation of Ba2+ from blocked channels is very slow, as ascertained by infrequent test pulses applied after washing Ba2+ form either inside or outside. The time constant for recovery from internal and external Ba2+ is the same. Frequent pulsing greatly shortens recovery time constant after washing away both Ba2+in and Ba2+out. Block by Ba2+ applied internally or externally is voltage dependent. Internal Ba2+ block behaves like a one-step reaction governed by a dissociation constant (Kd) that decreases e-fold/12 mV increase of pulse voltage: block deepens with more positive pulse voltage. For external Ba2+, Kd decreases e-fold/18 mV as holding potential is made more negative: block deepens with increasing negativity. Millimolar external concentrations of some cations can either lessen (K+) or enhance (NH+4, Cs+) block by external Ba2+. NH+4 apparently enhances block by slowing exist of Ba ions from the channels. Rb+ and Cs+ also slow clearing of Ba ions from channels. We think that (a) internally applied Ba2+ moves all the way through the channels, entering only when activation gates are open; (b) externally applied Ba2+ moves two-thirds of the way in, entering predominantly when activation gates are closed; (c) at a given voltage, Ba2+ occupies the same position in the channels whether it entered from inside or outside.